The present invention relates generally to gimbal control systems, and more particularly, to a gimbal control system that employs inner and outer control loops.
Conventional systems for spacecraft antenna control do not simultaneously use autotrack and resolver references. Many systems do not have resolvers at all and in the absence of autotrack sensor signals steer the antenna by open-loop counting of steps. Conventional antenna control systems that use resolvers for closed-loop control of gimbal position in the absence of an autotrack signal would, upon detection of the autotrack signal, commence to use that signal exclusively; that is, they would either use the autotrack or the resolver reference, but not together. This limits system performance by not using all the available sensor information.
Heretofore, a variety of gimbal control systems have been developed for use in controlling the pointing direction of satellite antennas. For example, a gimbal control system employed on the MILSTAR spacecraft has resolvers and autotrack receivers. However, they use one or the other, never both at the same time. The current-generation TDRSS satellite uses an autotrack receiver as a pointing reference for its gimbal. However, it does not have a resolver. When performing "program track" (or open-loop) pointing, it keeps track of the antenna's position by counting motor steps. Gross updates of the antenna's position are available from potentiometers, but are not used in any closed-loop algorithms. Satellites such as INTELSAT VI or AUSSAT-B use ground-based beacons as pointing references for their antennas. These beacon tracking systems are much like the TDRSS design, in that they do not have a resolver but keep track of antenna position by counting steps.
Other gimbal systems with very high performance requirements (such as those developed for laser pointing) often have a highly accurate high bandwidth sensor such as a gyro on the payload (i.e., mounted on the object that is steered by the gimbal). In these cases, that sensor (the gyro or other sensor on the payload) is the primary or only sensor used to control the gimbal. The gyro may be supplemented with a lower-bandwidth target tracking loop. This is the opposite of the design concept of the present invention, where a payload-mounted sensor is only used to provide low-bandwidth correction to a control loop using the resolver.
U.S. Pat. No. 5,062,592, granted to H. Kishimoto, issued Nov. 5, 1991, entitled "Orientation Control Apparatus for Space Vehicle", describes a spacecraft with a gimballed antenna. The antenna has an RF autotrack sensor to sense its inertial position. There is also a rate sensor for sensing the rate of the spacecraft main body (not the antenna), and both sensors are used simultaneously for controlling the antenna gimbal. This differs from the present invention because there is no sensor (such as a resolver) for measuring the relative orientation between the spacecraft and the antenna.
A number of papers relating to control of gimballed payloads have been presented at the annual SPIE Conference on Acquisition, Tracking, and Pointing. For example, a paper entitled "Design and Performance of a Satellite Laser Communications Pointing System," by R. Deadrick, Proc. 8th Annual Rocky Mountain Conference, Keystone, Colo., 1985, is an example of numerous papers that describe gimbal control systems with both resolvers and sensors for measuring payload pointing (quadrant detector in this case), but both sensors are not utilized simultaneously: the resolver is used only in acquisition mode, and the quadrant detector is used only in track mode. Another paper entitled "Acquisition and Tracking System for a Ground-Based Laser Communications Receiver Terminal," E. Clark & H. Brixley, SPIE Vol. 295, Control and Communication Technology in Laser Systems, 1981, pp. 162-169 describes a similar system.
A paper entitled "Attitude Acquisition and Tracking Capabilities of the Instrument Pointing System," by J. Busing and P. Urban, in the First SPIE Conference on Acquisition, Tracking and Pointing, April 1986, describes a control system for a gimballed telescope which simultaneously uses gyros, optical sensors, resolvers, and accelerometers. The optical sensor is used to calibrate gyro rate drifts, and thus the gyro is an inherent part of this control system. The detailed control architecture is not shown.
A paper entitled "Azimuth/Elevation Servo Design of the W. M. Keck Telescope," by M. Sirota and P. Thompson, in the Second SPIE Conference on Acquisition, Tracking and Pointing, January 1988, describes a system for controlling a gimballed telescope with simultaneous feedback of accelerometer, tachometer, and encoder measurements. No optical or RF reference is used.
A paper entitled "The Enhancement of Armored Vehicle Fire Control (Stationary and Fire-on-the-Move) Performance Using Modern Control Techniques," J. Groff, presented at the Third SPIE Conference on Acquisition, Tracking and Pointing, March, 1989, describes a system for controlling a gun turret using (simultaneously) a gyro, a tachometer, a potentiometer, and an optical gimbal angle encoder. The control compensation and architecture appear to be significantly different and more complicated than the present invention.
A paper entitled "A Low-Cost Alternative to Gyroscopes for Tracking System Stabilization," by D. Laughlin et al., in the Fourth SPIE Conference on Acquisition, Tracking and Pointing, April 1990, describes a gimbal control system using a gyro or magnetohydrodynamic device mounted on the payload for measuring angular rate, and closing a high-bandwidth inner "stabilization" feedback control loop, and simultaneously using an optical or RF sensor for closing a low bandwidth outer "track" feedback control loop. No resolver or other relative angle sensor is used. Several other papers describing the same configuration are found in the SPIE conference proceedings.
A paper entitled "A New Generation Control System for Ultra-Low Jitter Satellite Tracking," by W. Verbanets and D. Greenwald, in the Fifth SPIE Conference on Acquisition, Tracking and Pointing, April 1991, describes a gimbal system with simultaneous feedback of accelerometer and position encoder measurements. The compensation is different from the present invention and no inertial optical or RF sensor is employed.
A paper entitled "Optimization of Gimbal Scanned Infrared Seeker," by E. Williams, R. Evans, K. Brant, and L. Stockum in the Fifth SPIE Conference on Acquisition, Tracking and Pointing, April 1991, describes a control system for a seeker that simultaneously uses resolver and gyro feedback. However, the paper does not describe the control compensation.
A paper entitled "Universal Beam Steering Mirror Using the Cross Blade Fixture," by M. Meline, J. Harrell, and K. Lohnes, presented at the Sixth SPIE Conference on Acquisition, Tracking and Pointing, April 1992 includes a block diagram of a system for controlling a gimballed mirror which has an inner control loop using a measurement of the relative angle between the mirror and the basebody. However, this feedback loop has a lower bandwidth than the main outer optical control loop and has the express purpose of canceling the mirror control motor's back EMF, but does not provide any control of the mirror angle beyond that.
While several of the control systems for gimbals described above share certain characteristics with the present system, all of them are different in some fundamental way. The fundamental characteristics of the present system that do not appear in any of these papers are: (1) the simultaneous use of gimbal position measurements relative to both the spacecraft and the target (i.e., resolver and optical or RF tracking sensor measurements); the systems in the published references either do not use these measurements simultaneously, or use some other combination of measurements; and (2) the control filtering used to combine these measurements to account for biases between a programmed (open-loop) command reference and the measured target position from the optical or RF tracking sensor.
Accordingly, it is an objective of the present invention to provide for a gimbal control system that simultaneously uses measurements of relative and inertial gimbal position and a control filtering scheme to combine these measurements to account for biases between the programmed (open-loop) command reference and the measured target position from the optical or RF tracking sensor.